Characterization of Liquid-Pulp Fiber Suspension Mixing At Tee Mixers Using Electrical Resistance Tomography

Characterization of Liquid-Pulp Fiber Suspension Mixing at Tee Mixers using Electrical Resistance Tomography

W. Yenjaichon*, J.R. Grace, C.J. Lim and C.P.J. Bennington⁺

Department of Chemical and Biological Engineering

University of British Columbia

Vancouver, BC, Canada V6T1Z3

Mixing is an essential unit operation in the pulp and paper industry. In bleach plants, pre-distribution of chemicals in pulp suspensions before entering tower reactors is very important, and in-line mixers have normally been used to ensure efficient contacting between chemical and pulp in the tower to achieve the greatest lignin removal and optimal use of the bleaching chemical. Chemical injection in pulp mixing operations is commonly used as a pre-distributor to mix liquid chemicals into pulp suspensions ahead of various mixers including static mixers, peg mixers and high-shear mixers. The simplest type is the tee mixer. Understanding jet mixing behavior at a tee junction provides basic concepts for further analysis of different types of in-line mixers. However, few studies have been reported on the in-line mixing behavior of liquid injection into pulp suspensions. This presentation will focus on a study of mixing liquids into pulp fiber suspensions at tee mixers using a non-invasive technique “electrical resistance tomography (ERT)”.

The experiments were conducted in a pilot-scale flow loop facility. Mixing quality of liquid injection into pulp fiber suspension flow was evaluated after tee mixers for two pulp types (softwood and hardwood kraft pulps) over a range of suspension mass concentrations (C_m = 0 – 3%), main stream or pipe velocities (U_p =  0.5 – 5 m/s) and side stream or jet velocities (U_s = 1 – 12.5 m/s). The degree of mixing was determined from the measurement data in cross-sectional planes along the pipeline using a modified mixing index based on the coefficient of variation (CoV) of the individual conductivity values in each image pixel. The results show significant differences in jet mixing behavior of non-Newtonian pulp suspensions and a Newtonian fluid (water). For Newtonian fluid in the turbulent flow regime, mixing quality significantly improved with increasing jet-to-pipe velocity ratio. In addition, at the same jet-to-pipe velocity ratio, the jet penetration and mixing quality were almost independent of the main stream velocity for a specific jet-to-pipe diameter ratio.

For pulp fiber suspensions, on the other hand, the mixing quality strongly depends on the main stream velocity since the flow regime varies with the main stream velocity and the mixing quality of pulp suspensions strongly depends on the flow regime. At low velocity, plug flow was approached and mixing was very poor due to the strong fiber network, preventing tracer distribution downstream. At higher velocity, the plugs disintegrated, and the mixing quality significantly improved downstream in the pipe when the flow was in the turbulent regime. In addition, the mixing quality depended slightly on the jet velocity unless the jet momentum force was high enough to disrupt the fiber network and provide better mixing. At higher mass concentration (C_m ≥ 2%), mixing was very poor, even at a high main stream or jet velocity, since the turbulent shear was not high enough to disrupt the plugs. Therefore, additional shear was required to improve mixing.

* Author to whom correspondence may be addressed.

E-mail address: wyenjaichon@chbe.ubc.ca

⁺ Deceased